The present invention relates to a transmission line having extremely short signal propagation delay time.
Heretofore there has been proposed a transmission line as shown in FIG. 1. That transmission line 1 is made up of a signal conductor 2 placed at the center of the covering 4, a pair of conductors 3 placed on both sides of the signal conductor 2, and the insulating resin covering 4 such as polyethylene, which is called "form keeping resin material", having a rectangular cross-section. The signal conductor 2 and the conductors 3 are kept parallel to one another at a fixed transverse separation distance. The conductors 3 are either grounding wires for the signal conductor 2 or act as mechanical reinforcement. Only one other conductor may suffice in some cases.
The transmission line 1 of FIG. 1 may be used alone or it may also be used in multiple component cables. In the latter case, a plurality of transmission lines 1 are joined side-by-side by fusion bonding of the covering 4 to form a multiple flat cable 5 shown in FIG. 2. The distance between the signal conductors 2 is usually 1.27 mm.
The conventional transmission line mentioned above has disadvantages. It has a relatively long signal propagation delay time because the electromagnetic wave resulting from signal transmission concentrates in the covering 4 if made of polyethylene resin or the like, as is usual for the form keeping resin material. In the case of a transmission line as shown in FIG. 1, the propagation delay time is about 4.7 nsec/m, and it has heretofore been impossible to reduce it below 4.0 nsec/m for a transmission line of this kind. For the characteristic impedance desired, it is necessary that the conductors 3 be placed as far away as possible from the signal conductor 2. Such an arrangement reduces the thickness of the covering 4 in the vicinity of the surface 4a. This leads to insufficient dielectric strength when an electric current is applied to the conductor 3 while the transmission line is used underwater. Moreover, in the case of a multiple component flat cable, it is necessary to keep adjacent conductors 2 an adequate distance apart from one another.
The present device is intended to overcome the above-mentioned disadvantages inherent in the conventional transmission lines of this kind, and to provide a transmission line having improved transmission characteristics.